1. Field of the Invention
The invention relates to a photoresist trimming process, and more particularly to a photoresist trimming process incorporating oxide gases into the etchant.
2. Background Description
In photolithographic mask fabrication, it is typically preferable for the sidewalls of the mask to be as perpendicular to a reference surface as possible. Typically, the sidewalls are preferred to be perpendicular to the base surface of the substrate. However, an unwanted resist or mask foot or scum may form a generally sloping structure which is narrow at its top, and broadens towards its bottom. That is, the mask foot is an extension of mask material extending from a mask sidewall into the opening in the mask bounded by the sidewall.
A mask foot may typically protrude about 20 nm from a sidewall of a mask about 3000 Å thick. Where the sidewall bounds a trench in the mask on the order of about 500 nm, a 20 nm foot represents about a 10% error when the underlying layer is imaged using the mask, when considering that the opposite sidewall will have a similar mask foot. Such large mask errors complicate the design process and reduce manufacture yield.
Because the resist foot tends to be copied into any structure etched by the etchant, the foot typically forms a defect on the image photoresist. This defect is then replicated when the lower substrate is etched using the photoresist mask to guide the etching of the substrate.
Accordingly, methods have been developed in an attempt to remove and/or reduce the size of the resist foot. One method to reduce the size of the resist foot includes using a higher power level of radiation when exposing the photoresist, and/or using an etchant having higher selectivity to etch photoresist.
However, exposing the resist at higher power levels, and/or etching the resist with a more selective etchant typically damages that portion of the photoresist mask which is desired to be left intact. Such damage typically includes weakening the photoresist mask and thus reducing the ability of the photoresist mask to form precision or critical edges.